Field of the Invention
The present invention relates to a developing apparatus, which is suitable for an image forming apparatus using an electrophotographic technology, such as a printer, a copying machine, a facsimile machine, or a multifunction peripheral.
Description of the Related Art
An image forming apparatus such as a printer, a copying machine, a facsimile machine, or a multifunction peripheral includes a developing apparatus configured to develop an electrostatic latent image formed on a photosensitive drum with a developer so as to form a visible image. As such a developing apparatus, there is known a developing apparatus having a function separation type configuration including a developing chamber from which the developer is supplied to a developing sleeve and an agitating chamber in which the developer removed by the developing sleeve is collected, and the developer is circulated between the developing chamber and the agitating chamber (Japanese Patent Application Laid-Open No. 2004-205706 and Japanese Patent Application Laid-Open No. 2009-211087).
In the developing apparatus, a dual-component developer (hereinafter referred to simply as “developer”) containing a non-magnetic toner and a magnetic carrier is used. In this case, the toner is consumed during image formation, whereas the carrier is repeatedly used without being consumed. However, the carrier repeatedly used for a long period of time is degraded in charging performance for the toner. When the developer containing a large amount of such carrier is continuously used, an image defect such as a density unevenness or fogging is liable to occur. Therefore, in order to replenish approximately the same amount of toner as the amount of consumed toner and to suppress the degradation of the carrier, there is known an apparatus using a carrier refresh (ACR: Auto Carrier Refresh) process of replenishing a supplement containing the toner and the carrier and discharging a surplus developer (Japanese Patent Application Laid-Open No. S59-100471).
In the case of the developing apparatus of the function separation type, when the developer is brought into a stable state (so-called steady state), a surface level of the developer becomes higher from upstream toward downstream in a direction in which an agitating screw conveys the developer. Thus, the developer is liable to accumulate downstream of the agitating chamber, that is, upstream of the developing chamber. Therefore, when the ACR process is used for the developing apparatus of the function separation type, a discharge port configured to discharge the surplus developer is formed downstream of the agitating chamber. In the related-art developing apparatus of the function separation type using the ACR process, however, even when the amount of developer is relatively small, the developer is liable to be continuously discharged. Thus, with the related art, the amount of developer is excessively reduced, and a sufficient amount of developer cannot be supplied to the developing sleeve, resulting in occurrence of the image defect.